Flower development in Pisum sativum: From the war of the whorls to the battle of the common primordia

The ontogeny of pea (Pisum sativum L.) flowers, as in many legume and nonle gume plant species, proceeds through a very different sequence of events fr om the same process in Antirrhinum majus and Arabidopsis thaliana. Using sc anning electron microscopic analysis, we have characterized the early devel opment of wild-type pea flowers and selected morphological characters or ma rkers to break it down into different developmental stages. We used these m arkers as tools to characterize early alterations in flower development of several pea floral homeotic mutants. These mutants display phenotypes resem bling those of: (1) floral me ristem identity mutations, frondosus (brac); (2) class A mutations, calix carpellaris (cc); (3) class B mutations, stami na pistilloida (stp-1 and stp-2); and (4) class C mutations, petalosus (pe) . According to the homeotic transformations observed in the pea floral muta nts, it would appear feasible that the identity and developmental pattern o f the four organ types in pea flowers are governed by at least the same thr ee developmental functions, A, B, and C, proposed for the two model systems . However, our results suggest that, in pea, although these functions do ha ve a similar role in the specification of organ identity shown by their cou nterparts in Arabidopsis or Antirrhinum, they may differ in the control of other processes, such as floral determinacy, organ number, or leaf developm ent. The more remark able features of pea flower ontogeny were the existenc e of four common primordia to petals and stamens, the early carpel primordi um initiation, and the abaxial-adaxial unidirectional initiation of organ p rimordia within each different floral whorl, in contrast to the centripetal and sequential floral ontogeny in other plants. Organ differentiation with in each of these common primordia appears to be a complex process that play s a central role in he ontogeny of pea flowers. Analysis of flower developm ental pea homeotic mutants suggests that A, B, and C functions are necessar y for the correct differentiation of organs from common primordia and that, in addition to its role in the specification of petals and stamens, B func tion, would be involved in conferring common primordia identity. (C) 1999 W iley-Liss, Inc.